Since 1991, the notion of first spoken official language has been used. In fact, this concept was derived from questions which already were part of the census, namely knowledge of the official languages, the mother tongue, and the language spoken at home. The notion of first spoken official language is used by many people in the community because it takes into account those people for whom the mother tongue is neither French nor English.
For example, Canada has taken in over one million new immigrants over the past five years. Now, 80% of these people don't have French or English as their mother tongue. As a rule, the term "allophone" was used to designate those people who spoke another language, however, increasingly, people speak English from a young age. Even though English isn't their mother tongue, they consider themselves anglophones. In the same way, outside Quebec, immigrants from Algeria or Morocco whose mother tongue is Arabic consider themselves francophones because they use French on a daily basis. And that is why Statistics Canada has not imposed any definition of the word "francophone".
I am aware that there is a lot of debate over the criteria used to define francophones. Some people believe that a francophone is someone who speaks French at home, for others it's the mother tongue, and for some it's the first spoken official language.